PLEISTOCENE. 49 



Perhaps of somewhat similar age is the brickearth at the 

 Upminster Pottery and brickyard, to the east of Martin's and 

 north of Upminster Hall ; a deposit which occupies a small area 

 from 144 to about 150 ft. above O.D. It is about 20 ft. thick 

 in the centre and occupies a hollow in Chalky Boulder Clay. 

 Small patches of gravel are associated with the Boulder Clay of 

 the north-east corner of our area. 



South of the Thames undoubted glacial gravels containing 

 Bunter pebbles are found only in one district; there are now 

 three patches, in Richmond Park, on Kingston Hill and on 

 Wimbledon Common, separated by the valleys of the Beverley 

 Brook and Pen Ponds ; but these must once have formed a 

 continuous sheet. The finest section is at Coombe Warren, 

 Kingston Hill, where the Urban District Council's pit has a face 

 a quarter of a mile long and 8 to 20 ft. high. The total thickness 

 is about 25 ft. The gravel consists mainly of southern material, 

 including subangular blocks of resinous chert, pebbles of spongy 

 chert, ironstone, sarsens, Chalk flints, some of them fresh and of 

 great size, Tertiary pebbles and thick seams of sand which appear 

 to be almost entirely Bagshot. Besides these, there is some 

 10 per cent, of northern material, such as quartz, quartzite, 

 igneous rocks, Carboniferous cherts, veinstones etc. ; the quartz- 

 ite pebbles are sometimes as much as 9 in. long. The deposit 

 is strongly current-bedded and, in the upper 6 ft., much contorted. 



This pit is near the present southern limit of the gravel; at 

 the northern end a small pit in Sidmouth Wood, Richmond Park, 

 shows a markedly different composition; the same components 

 are all present, but northern material forms more than half of 

 the total. 



On Wimbledon Common the gravel is more shingly, that is 

 to say, there is less fine material, but this may have been leached 

 out of the gravel by the copious springs, leaving only the pebbles ; 

 these are smaller and more angular than at Coombe, and the 

 proportion of northern material does not exceed 10 per cent. 



CHAPTER VIII. 



PLEISTOCENE (continued). 



RIVER DEPOSITS. 



The Lower Thames Valley as we now see it is a broad tract 

 of gravel and brickearth, traversed by the main river and inter- 

 sected by numerous tributary streams which flow from north 

 and south. 



The deposits all bear evidence of having been distributed by 

 fluviatile action, but under conditions different from those which 



